1. Field of the Invention
The present invention relates to an improved suspension for a vehicle, and more particularly, to a self-propelled vehicle having bogie-type running gear.
2. Description of the Prior Art
The present invention relates to Canadian patent application No. 539,382, filed June 10, 1987, naming Jean-Marie Cote as inventor, and having a common applicant with the present application. This earlier filed application describes a bogie-type, self-propelled vehicle having a chassis with a bogie on each side of the chassis. The vehicle has a hydraulic system that includes actuator means connected between each bogie and the chassis for maintaining the chassis generally level on level ground. The hydraulic system includes first and second connecting systems extending between the two actuator means in a manner to have movement of one wheel in one bogie in one direction tend to cause movement of the corresponding wheel in the other bogie in the opposite vertical direction to the one direction to minimize tilting of the chassis. The improved suspension system also includes a second hydraulic system that can be selectively connected to one of the first and second connecting systems to tilt the chassis relative to the bogies in one direction or the other so as to generally maintain the chassis level on ground that slopes in the longitudinal direction of the vehicle.
It has been contemplated and, in fact, application No. 539,382 is an improvement over earlier mechanically linked bogie systems such as described in U.S. Pat. Nos. 3,313,555, issued Apr. 11, 1967, and 3,504,928, issued Apr. 7, 1970, to W. E. Reimer. In those patents, a bogie including a walking beam with a wheel at each end is centrally pivoted to each end of an axle extending transversely at the longitudinal center of the vehicle. A transverse rocking bar is pivoted to the front of the vehicle and to each end of the walking beams. In this manner, vertical displacement of any wheel causes an opposite vertical displacement of the corresponding wheel on the other side of the vehicle, ensuring four-point engagement of the wheels over any off-road terrain. Such advantages are also well known from U.S. Pat. No. 4,324,304 relative to a track or crawler-type vehicle. This last-named patent issued Apr. 13, 1982 to H. Hashimoto et al and is assigned to Mitsubishi Jukogyo K.K.
It has been found that the location of the rocking beam or bar in the Reimer patents as well as in the Mitsubishi patent is not suitable to off-the-road vehicles of the type used in forestry operations. The terrain in such operations is usually riddled with stumps and boulders, broken tree trunks and branches. It is important to design the underside of the chassis with a minimum of obstructions and to avoid locating any mechanical linkages or other elements which might be damaged or otherwise impede the movement of the vehicle over such rough terrain. However, the rocking beam or bar described in the prior art is located between the walking beams and thus in the lower part of the chassis where it creates an obstruction.